Lilly Collins as Clary in a scene from "The Mortal Instruments: City of Bones." (AP Photo/Sony Pictures Screen Gems, Rafy)

A young adult fiction binge has broken out in “Mortal Instruments: City of Bones.”

Like an 80-car pill-up, all of the current tropes of teenage fantasy here careen into one another: the young heroine with previously unknown powers; the gothic mix of heavenly and hellish supernatural creatures; the breathless romance with a young Brit of angelic cheekbones. And, oh, the tattoos.

It’s an overdose of mysticism, concocted to give devoted young fans their fix: a heartthrob to swoon over and grand battles to match inflated teenage emotions.

The film is based on the first in a series of popular young adult novels by Cassandra Clare, whose writing originated in “Harry Potter” fan fiction. It’s a blatant inspiration to “City of Bones,” the first of a planned franchise, as is (if you haven’t already guessed) “Twilight.”

Like a hand bag bought on a New York street, this is the knockoff version.

Lily Collins stars as Clary, a teen who discovers that her mother (Lena Headey) is secretly a Shadowhunter, a hunter of demons. At the same time as her mom is kidnapped, Clary, a bright redhead, realizes she’s able to see a hidden world in their native New York, one where Shadowhunters, demons, werewolves and warlocks stealthily operate in varying degrees of gothic drab, invisible to humans, or “mundanes.”

It’s a lot like a mediocre episode of “Buffy the Vampire Slayer.”

As she hunts for her mom, Clary is quickly indoctrinated into this underworld, led into it by the aforementioned heartthrob, Jace (Jamie Campbell Bower), an alluring Shadowhunter with blond locks.

Directing from Jessica Postigo’s overstuffed screenplay, Harald Zwart (the “Karate Kid” remake) summons the kind of dingy, baroque, nighttime atmosphere that Jonathan Rhys Meyers revels in. As if falling for the trap, he reliably turns up to lend his pallor to the proceedings as the villain Valentine. He’s seeking a magical cup that Clary might — if she can stir the memories her mother had tried to block — know the location of.

The plot is astonishingly overcooked: a mishmash of watery portals, Greek-styled father-son drama, invisible buildings and supernatural battle lines. It reaches an apogee of ridiculousness when, we are informed, Bach was a Shadowhunter, whose compositions were designed to ward off demons.

What can be said for “City of Bones” (a sequel is already in the works) is that it’s not brooding or abundantly self-serious. The cast generally lends enjoyable performances, even if the filmmaking (a made-for-TV movie level of craft, even with the abundant special effects) is lacking.

The handful of comic, self-deprecating moments in the film — quick nods to the mounting silliness — gives “City of Bones” a pleasant cheeriness that it would have done well to pursue further. Instead, it desperately gathers clichés like a teen fantasy Frankenstein that doesn’t realize its secret identity: parody.

“Mortal Instruments: The City of Bones,” a Sony Screen Gems release, is rated PG-13 by the Motion Picture Association of America for “intense sequences of fantasy violence and action, and some suggestive content.” Running time: 130 minutes. One and a half stars out of four.

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MPAA rating definition for PG-13: Parents strongly cautioned. Some material may be inappropriate for children under 13.